Added system-wide minimum brightness curve.

The minimum brightness curve guarantess that any brightness curve
that dips below it is rejected by the system.
This prevent auto-brightness from setting the screen so dark as to
prevent the user from resetting or disabling it, and maps lux to
the absolute minimum nits that are still readable in that ambient
brightness.

Test: atest BrightnessConfigurationTest.

Fixes: 77176207

Change-Id: Ibd1e83e9b147f3849d6c907f828cbe5950c8367f